H-Bridge control DC motor

Hello All,
I am using a L6203 single H-Bridge driver and PWM to control a dc motor which rotates a flat bed clockwise or anticlockwise as selected. When driven from a bench top power supply at a voltage of 30v, the current drawn is between 200mA to 400mA depending on the PWM frequency. The current PWM settings are 1ms on (H), 9 ms off (L). The flatbed position is obtained by using a digital potentiometer connected to the centre of the table. As the table rotates through 360 deg so does the pot. The pot output is approx 50mv at 0 deg rising linearly to about 4.6v at 360 degrees. Each degree of rotation being approximately 12.6mv.
The problem is that as the H-bridge is driven, it creates spikes on the dc supplies (30v motor supply and 5v logic suppy) even though they are heavily filtered. These spikes feed back to the pot output which messes up my ADC reading and so the position of the table is lost.
The dc motor has been replace by an 8 ohm resistive load and the spikes remain.
I would be grateful for any help or advice.
Cheers
Paul

Hi Paul,

Two things come to mind. Your powersupply may not be able to handle the pulsating current. A beefy capacitor should take care of that, try a 10000uF/35V electrolythic and see if that helps. It may also be that you have weak and/or common ground wires for your pic and motor. They should be separate, heavy and short.

Hello Ingvar,
That the value I have in circuit already.
Thanks Paul